This invention relates generally to combustors for gas turbine engines and more particularly to properly and repeatedly positioning all pilot fuel assemblies in such combustors.
A gas turbine engine includes a compressor that provides pressurized air to a combustor wherein the air is mixed with fuel and ignited in a combustion zone for generating high temperature gases. These gases flow downstream to one or more turbine stages that extract energy therefrom to power the compressor and provide useful work such as powering an aircraft in flight or land based engines. It is desirable to reduce exhaust emissions produced by the combustion process. This is particularly true for the new generation of supersonic transport, referred to as High Speed Civil Transport (HSCT), that is currently under development. For HSCT to be viable, emission levels, particularly NOX, must be significantly reduced relative to present aircraft while maintaining high combustion efficiencies. Efforts to reduce emissions in the HSCT have led to the development of a combustor having a plurality of pilot fuel systems that direct a swirled mixture of fuel and air radially into the combustion zone.
Such pilot fuel systems typically include a pilot fuel assembly and a swirler assembly. A major assembly requirement for this type of pilot fuel system is to maintain concentricity between the pilot fuel assembly and its corresponding swirler assembly. Since the air exiting from the swirler assembly is swirling at a defined angular discharge, a static pressure imbalance can develop within the swirling chamber if the concentricity is not maintained. This results in local static pressure variations that can cause the fuel/air mixture to pre-ignite prior to its discharge into the combustion chamber. This is a potentially hazardous condition in locations where combustion is undesirable. However, because it is mounted from outside of the outer combustor casing, installation of the pilot fuel assembly is a blind process. Thus, concentricity between the pilot fuel assembly and the swirler assembly cannot be maintained or even determined. In addition, concentricity variations from one pilot fuel system to another will exist, resulting in non-conformance in minimizing pre-combustion. The problem is compounded by the dimensional stack-up for the various pilot fuel system components that results from the manufacturing tolerances.
Accordingly, there is a need for a repeatable method of providing concentricity for a pilot fuel system in a gas turbine combustor. There is also a need for a combustor having a pilot fuel system configured in such a manner that permits concentric installation and minimizes concentricity variations.
The above-mentioned needs are met by the present invention which provides a gas turbine combustor having a combustor casing with an opening formed therein and a boss formed thereon surrounding the opening. A pilot fuel assembly having a mounting flange is disposed in the opening, with the mounting flange being encircled by the boss. Both the boss and the mounting flange are provided with at least two flat surfaces that are perpendicular to each other. The pilot fuel assembly is concentrically installed to the combustor casing by inserting the assembly into the combustor casing opening, and moving the pilot fuel assembly as far as it will go in a first direction substantially parallel to one of the flat boss surfaces. The distance between the other flat boss surface and one of the flat flange surfaces is then taken. Next, the pilot fuel assembly is moved in the direction opposite the first direction, at which point, the distance between the same two flat surfaces is again measured. Lastly, the pilot fuel assembly is located at a position where the distance between the two measuring surfaces is equal to the average of the first and second measurements. If desired, these steps can be repeated back and forth along an axis perpendicular to the first and second directions.
Other objects and advantages of the present invention will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.